Radar pulse compression system

ABSTRACT

A pulse compression system comprises in association with a magnetron, or other similar oscillator, transmission line means and means for accelerating an effective short circuit along said transmission line means towards the magnetron or other oscillator in order to produce a Doppler shift in the reflected wave before it is fed to an aerial coupled to said transmission line means. The effective short circuit may be provided by an ionized gas plasma generated within a waveguide or coaxial cable and which may be accelerated along the tube at the requisite rate to produce the desired Doppler shift by applying suitable magnetic and electric forces.

United States Patent Inventor Michael G. T. Hewlett Ilford, England Appl. No. 881,360 Filed Dec. 2, 1969 Patented Oct. 5, 1971 Assignee The Plessey Company Limited Ilford, Essex, England Priority Dec. 6, 1968 Great Britain 57982/68 RADAR PULSE COMPRESSION SYSTEM [56] References Cited UNITED STATES PATENTS 3,048,838 8/1962 Bretscher 343/5 X 3,396,388 8/1968 Goldie 343/5 2,795,698 6/1957 Cutler 343/l7.2 PC

Primary Examiner-Malcolm F. l-lubler Anomey- Blum, Moscovitz, Friedman & Kaplan shift in the reflected wave before it is fed to an aerial coupled 4 Claims 1 Drawing to said transmission line means. U.S. 343/5 R, The efiective short circuit may be provided by an ionized 343/ 17.2 PC, 343/ 17.5 gas plasma generated within a waveguide or coaxial cable and Int. Cl. G0ls 9/02 which may be accelerated along the tube at the requisite rate 343/5, 17.2, to produce the desired Doppler shift by applying suitable magi7.2 PC, 17.5 netic and electric forces.

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2 MAGNET/C 6 5+ FIELD GAS 7 INJECTION AER/AL PATENTEDUBT 5m 7 0 TO l AER/AL RADAR PULSE COMPRESSION SYSTEM This invention relates to radar systems and is specifically concerned with radar systems employing so-called pulse compression, that is to say systems in which the frequency of the transmitted pulse varies, usually linearly, throughout the pulse.

' Pulse compression as used in radar systems is a technique by which the peak transmitted power normally required to ensure good range resolution and to assist in the discrimination between targets in a distributed clutter environment can be reduced to a relatively low value. For example, a peak transmitted power of 150 megawatts would normally be required in an 'S-Band radar system having a mean power of 5 kilowatts and using pulses of 0. l-microsecond duration with a pulse repetition rate of 330 p.p.s. By arranging that the pulse compression system affords a compression ratio of 50:1 the peak power can be reduced to 3 megawatts. Pulse compression systems, however, usually entail the provision of driven amplifier transmitters and by reason of this the cost of such systems is considerably greater than systems in which fixed frequency pulses obtainable from magnetrons or other relatively cheap power oscillators are utilized.

The present invention seeks to reduce the cost of pulse compression systems by providing in association with a magnetron or other similar oscillator transmission line means and means for accelerating an effective short circuit along said transmission line means towards the magnetron or other oscillator in order to produce a Doppler shift in the reflected wave before it is fed to an aerial coupled to said transmission line means.

For the provision of said effective short circuit associated with the transmission line means an ionized gas plasma generated within a waveguide of coaxial cable may be accelerated along the tube at the requisite rate to produce the desired Doppler shift by applying suitable magnetic and electric forces. The plasma behaves as an efficient low-loss reflectorof incident microwave energy.

By way of example the present invention will hereinafter be described with reference to the accompanying schematic drawing of a pulse compression system for use in radar.

As has been previously mentioned, a pulse compression ratio of 50:1 will be required in a S-band radar system in order to reduce the peak power say from 150 megawatts to 3 megawatts. To achieve this the product of the transmitted pulse length and the bandwidth of the frequency change in the pulse must be 50. Consequently if the transmitted pulse length is 5 microseconds the frequency change or bandwidth over the pulse will be 10 megahertz and this may be a linear frequency change or sweep for the pulse duration.

in the system of the present invention to be described later an effective short circuit is accelerated along a transmission line towards a magnetron in order to produce a Doppler shift in the frequency of the wave reflected from the short circuit. The terminal velocity of such a short circuit along the transmission line will be given by V=fdc/2j8.

Where fd Doppler shift; velocity of propagation; and f8 transmitted frequency.

Thus to provide a lO-megahertz Doppler shift in a 3,000 megahertz transmitted signal the terminal velocity is given by taking C as being 3Xl0cms./sec. which is the velocity of propagation in free space.

Provided the short circuit to be produced in the line accelerates at a constant rate from rest at the start of the transmitted pulse to the terminal velocity mentioned above the reflected wave will experience a linear frequency change or sweep of 10 megahertz during the pulse.

Assuming that the pulse has a microsecond duration then the short circuit will travel during this. period a distance given y 5 X ems/sec.

Where u initial velocity; b final velocity; and t time. The distance travelled can thus be shown to be centimeters. Consequently, the short circuit will need to travel 125 centimeters along the transmission line so that it reaches a velocity of 5X10 centimeters per second.

Electromagnetic energy having an oscillating frequency will be totally reflected if f=Ne /41r G,m. where N number of electrons/cubic meter e electron charge (1.59x10" Coulombs) m electron mass (9Xl0' kg.)

G, dielectric constant of free space.

From the formula just set forth the electron density required at 3,000 megahertz can be found to be l'lXl0per cubic centimeter. This is reasonably small and can be achieved by the use of a plasma device as included in the system now to be described.

Turning then to the system illustrated in the accompanying drawing this includes a plasma device indicated generally at l and includes a tube or waveguide 2 which is connected via a waveguide 3 to the output side of a magnetron 4. The magnetron is pulsed in conventional manner from a modulator 5 which also feeds a proportion of the voltage to the plasma device 1.

In operation of the system, prior to each pulse output of 3,000 megahertz from the magnetron a mass of gas (hydrogen) is injected into the plasma tube or waveguide 2 through a hollow electrode 6 to which a proportion of the magnetron modulator pulse voltage is applied as previously mentioned. An alternative arrangement would be to use a dispensing electrode rather than a gas injection electrode system. The injected gas ionizes and since its axis is normal to an applied magnetic field extending transversely across the tube 2 it will accelerate up the tube towards the magnetron 4. The field strength will be kept constant as will be the gas column current so that the constant force exerted on the gas will produce a constant acceleration of the ionized gas.

During this operation the magnetron output pulse incident on the accelerating plasma will be continually reflected from it and will be coupled out of the transmission line by a coupler or circulator 7 into an aerial. The acceleration of the plasma which acts as a low-loss reflector of the pulse energy produces a Doppler shift in the pulse frequency. It may be necessary to extend the pulse electrode 6 down the transmission line to maintain ionization of the gas throughout its travel. The modulator 5 must in the system described be capable of supplying the power dissipated in the plasma as well as that required by the magnetron 4. The plasma arc power should be relatively small but will appear as a system loss. Continual evacuation of the plasma tube or waveguide 2- is likely to be required in order to prevent excessive plasma cooling and also to prevent uncontrolled spread of ionization and for this purpose the tube 2 is closed at the end thereof nearest the magnetron with a vacuum window 9 and the tube 2 can be evacuated through part 10 by the operation of a vacuum pump (not shown). Alternative contemplated forms of the device involve the use of a coaxial transmission line in which the plasma is formed as a radial annular reflector which moves under the influence of a magnetic field formed by passing current down the center conductor of the line.

In establishing the approximate field strengths and other data which will be required certain assumptions may be made but since the plasma terminal velocity is several orders down on near relativistic velocities it is possible to apply classical electromagnetic and mechanical laws.

In this particular case under consideration the terminal velocity of 5Xl0"7 cm./sec. will require a constant acceleration a of B flux density 100 gauss I arc current in amps 100 amps I narrow dimension of guide 3.5 cms.

l l00 3.5 3.5 X dynes F: 10

Now this accelerating force can be related to the (mass at acceleration) figure to establish the maximum possible mass of gas for the parameters chosen:

Thus

3.5 X l0 l0 m 3.5x 10 m- 10 gms or, since m where w= weight of gas grams 3.5 X 10- grams If hydrogen is assumed to be used as the working plasma material the following conditions apply:

density of hydrogen (H at N.T.P. =9 lO'-" grams/cc. Therefore, 3.5Xl0" grams would represent a volume at N.T.P. of

3.5 l0- l0 4 l0--" cc cc If it is assumed that the required hydrogen operating pressure is of the order of 0.5 mm. Hg in the arc space, then from P V=K, the presence of 4X10 cc. (N.T.P.) hydrogen would occupy a volume of 6.0 cc. Taking a plasma column length of 3.5 cm., this would provide a diameter of about 1.5 cm.

As will be appreciated from the foregoing the plasma device produces a Doppler shift in the transmitted pulse from the magnetron and thereby obviates the need for expensive and complex driven amplifier equipments.

What we claim is:

1. A pulse compression system for use in radar, comprising a high-power high-frequency oscillator, transmission line means connected to the output from said oscillator, and means for accelerating an effective short circuit along said transmission line means towards the oscillator in order to produce a Doppler shift in the reflected wave before it is fed to an aerial coupled to said transmission line means.

2. A pulse compression system as claimed in claim 1, in which the oscillator is a magnetron.

3. A pulse compression system as claimed in claim 2, in which the effective short circuit is provided by an ionized gas plasma generated within a waveguide or coaxial cable and accelerated along the waveguide by the energization of means for producing suitable magnetic and electric forces to influence said plasma.

4. A pulse compression system as claimed in claim 3, in which the magnetron is pulsed from a modulator which also feeds a voltage to the plasma device for the acceleration of the plasma along the waveguide. 

1. A pulse compression system for use in radar, comprising a high-power high-frequency oscillator, transmission line means connected to the output from said oscillator, and means for accelerating an effective short circuit along said transmission line means towards the oscillator in order to produce a Doppler shift in the reflected wave before it is fed to an aerial coupled to said transmission line means.
 2. A pulse compression system as claimed in claim 1, in which the oscillator is a magnetron.
 3. A pulse compression system as claimed in claim 2, in which the effective short circuit is provided by an ionized gas plasma generAted within a waveguide or coaxial cable and accelerated along the waveguide by the energization of means for producing suitable magnetic and electric forces to influence said plasma.
 4. A pulse compression system as claimed in claim 3, in which the magnetron is pulsed from a modulator which also feeds a voltage to the plasma device for the acceleration of the plasma along the waveguide. 